High-speed signaling systems often transmit data without a dedicated clock. Digital receivers must therefore have a reference clock or recover the clock from the data itself. This timing information is used to control a variety of functions in the receiver including the sampling process used to convert received data into digital signals.
A variety of recovery circuits have been proposed for extracting clock information from received data. A variety of supplied methods for the reference clock also exists. In general, due to issues with spectral purity, the reference clock will be provided at a lower rate and multiplied in frequency with an internal clock multiplication circuitry. These circuits often use an interpolator to set the phase of the recovered clock during sampling. From a performance standpoint, the location of the clock phase relative to an eye diagram generated for the data is a relevant consideration.
Eye diagrams are usually generated to determine the modulation characteristics of received data signals. The shape of an eye diagram changes depending on a number of receiver-related factors including noise margin, tolerances for timing and amplitude variations, and others generally, the width of the eye determines the timing margin and the widths of the eye transition points determine jitter. Also, a direct correlation exists between eye shape and the type of logic used to construct the receiver circuits. For example, current mode logic has a tendency to produce an irregularly shaped eye, whereas other logic types produce eyes with greater symmetry.
In order to recover clock phase information in a receiver, the incoming data is usually demodulated into in-phase and quadrature signals. The phase of the in-phase signal is then shifted to coincide with a crossing point in the eye diagram. Because the in-phase and quadrature signals are maintained in an orthogonal relationship, shifting the phase of the in-phase signal will necessarily re-position the quadrature signal phase 90E from the crossing point in the eye diagram. The position of the quadrature signal phase has a direct influence on the bit-error rate of the receiver. If this position is not optimum, bit-error rates will increase, which, in turn, will cause errors to be induced during the data sampling process.